Recently, the technology related to "silicon micromachining" has been developed into a family of new applications for making "microelectromechanical systems (MEMS)". This evolving technology has attracted great attention, and is considered another commercially explosive main stream after the semiconductor industry. It has been documented in the art that a diaphragm can be formed on a silicon macrostructure. The diaphragm so formed, or other diaphragm-derived components such as a diaphragm-sealed chamber or a diaphragm-based suspending arm, can be widely used in the industry for manufacturing pressure transducers, microvalves, actuators, accelerometers, shear-stress sensors, etc.
FIG. 1 is a schematic illustration of the steps of one of the conventional methods utilizing the bulk micromachining technique to fabricate a pressure sensor. FIG. 1 shows that a silicon sensing wafer 1 is bonded to a silicon constraint wafer 2 via a SFB (silicon :fused bonding) bond. The surfaces of the wafers to be bonded together have been treated to become hydrated surface so as to effectuate the wafer bonding therebetween. Prior to the bonding step, the silicon constraint wafer is formed with an anisotropically etched cavity 3. Thereafter, the silicon sensing wafer is etched to form a diaphragm 4 having an etched-back surface 8, opposite the fused interface 5. Ion-implanted piezoresistors 6 are formed on the diaphragm 4. Finally, the silicon constraint wafer is ground and polished for final wafer thickness. The conventional method requires a silicon wafer bonding step, which requires that the surfaces of both wafers be extremely clean. Furthermore, two polishing steps are required in the conventional method. Typically, an etch stop 7 is required to ensure the etching of the silicon sensing wafer to a predetermined thickness. This further adds to the cost for manufacturing semiconductor sensors.
As discussed above, one of the disadvantages of the silicon micromachining processes in making diaphragms for use in microelectromechanical systems is that the silicon wafers to be bonded must have extremely clean surface. Additionally, the conventional silicon micromachining processes may also involve the additional steps of forming an etching stop and performing a double-sided aligning procedure, which is a complicated and expensive procedure. Therefore, it is desirable to develop new silicon micromachining processes for making diaphragms which would require simplified fabricating procedure and is compatible with many other IC manufacturing processes and/or applications.
The following patents, whose contents are expressly incorporated herein by reference, provide some background information which may be relevant to the present invention.
U.S. Pat. No. 5,307,684 discloses generally a pressure transducer for measuring the pressure of a fluid in which a diaphragm capable of displacement is located within a cavity provided in a housing. The '684 patent further discloses a stop mechanism for increasing; the bursting pressure limit of the diaphragm for protection against calibration damage.
U.S. Pat. No. 4,664,762 discloses a method of electrochemically etching a silicon substrate of a diaphragm type silicon pressure sensor by etching one layer of a dual-layer silicon substrate having a first N-type silicon layer and a second P-type silicon layer. The substrate is first placed in an etchant and an electrode is formed on each of the first and second layers. A positive terminal of a voltage source is connected to the electrode of the N-type silicon layer, and a switch is provided which selectively connects the positive terminal to the P-type silicon layer. When the etching of the P-type silicon layer is nearly complete, the switch is opened to disconnect the P-type silicon layer from the positive terminal.
U.S. Pat. No. 5,310,441 discloses a method for automatically binding a silicon wafer to a carrier plate, which is used in a silicon wafer polishing apparatus. The silicon wafer is a monocrystal silicon wafer, and a fixed carrier plate is bonded to a plurality of wafer sheets. The carrier plate is casted into a polishing apparatus to carry out the polishing of the monocrystal silicon wafer. Typically wax is applied to the inside of the wafer as a bonding agent.
U.S. Pat. No. 5,189,591 discloses a capacitive pressure transducer which is made of aluminosilicate glass or any other glass having a low thermal coefficient. The capacitive pressure transducer comprises segments of aluminosilicate glass. One of the segments is first shaped into a pressure diaphragm, and a first electrode is formed on the surface of the pressure diaphragm. Then another segment is shaped into a second diaphragm and a second electrode is formed on the surface of the second diaphragm. Finally the pressure and second diaphragms are bonded together such that the first and second electrodes are formed into a capacitor.
U.S. Pat. No. 4,872,945 discloses a process for manufacturing the pressure transducer of a pressure sensor. A capacitive pressure transducer is first formed by bonding a silicon diaphragm to a glass base such that a cavity is formed between the diaphragm and the base, and the transducer produces different capacitances in response to different diaphragm deflections. The sensitivity of the capacitive pressure transducer is adjusted by etching the silicon diaphragm while it is bonded to the base substrate in accordance with capacitance values of the transducer. The thickness of the silicon diaphragm can be selectively reduced to obtain an accurate desired sensitivity for the transducer.
U.S. Pat. No. 5,242,863 discloses a method for fabricating a pressure sensor, which is made up of a substrate of which there is a diaphragm at or near the surface of the substrate with a chamber under the diaphragm, by first forming a low resistance N-type silicon layer buried under an overlying layer designed to be a diaphragm, and forming a low resistance N-type :material filled trench which penetrates the overlying layer and connects with the low resistance N-type silicon layer. The low resistance N-type material and the low resistance N-type silicon layer are converted into porous silicon by anodization of the silicon. Then the porous silicon is removed by an etching step. Finally the openings in the trench which are, formed after the removal of the porous silicon are filled with a sealing material to form a sealed reference chamber.
U.S. Pat. No. 5,177,661 discloses method for the fabrication of diaphragm pressure sensors utilizing the silicon-on-insulator (SOI) technology where recrystallized silicon forms a diaphragm which incorporates electronic devices used in monitoring pressure. The diaphragm is alternatively comprised of a silicon nitride having the necessary mechanical properties with a recrystallized silicon layer positioned thereon to provide pressure sensor electronics. In this method, a compliant membrane is formed that encloses a sacrificial insulating material. The insulating material is subsequently removed through an opening in the membrane to form a pressure sensitive diaphragm. Electrical elements are positioned in a single crystal silicon layer formed on or in the diaphragm to detect movements thereof and produce electrical signals proportional to the diaphragm displacement. The single crystal layer is formed by depositing a polycrystalline silicon layer and then zone-melt recrystallizing the film to form a high quality single crystal silicon suitable for CMOS circuitry.
U.S. Pat. No. 5,279162 discloses a semiconductor sensor containing an elastically deformable member, a field-effect transistor formed in a piezoelectric semiconductor material supported on the elastically deformable member, means for supplying a gate of the field-effect transistor with a predetermined DC bias, and a constant-current supply means for supplying a predetermined drain current to a drain of the field-effect transistor. In this invention,, semi-conductors of high piezoelectricity, such as GaAs, etc, are used in place of the conventionally used silicon.
U.S. Pat. No. 5,335,550 discloses a semiconductor pressure sensor containing a flat thin diaphragm formed by bonding a first silicon substrate and a second silicon substrate with an interface insulating film interposed therebetween. A circuitry including gauge resistors is fabricated on the primary surface of the second silicon substrate. The interface insulating film is disposed in the recess of a vacuum chamber and has a two-layer structure..Alignment marks are formed so as to allow the circuitry to be accurately formed relative to the vacuum chamber.
U.S. Pat. No. 5,245,504 discloses a method for manufacturing hinged diaphragms for semiconductor sensors, such as accelerometers, pressure transducers, etc, from a SIMOX wafer, in which an internal insulating silicon dioxide layer is used as an etch stop in removing silicon from the underside of the wafer by etching with an appropriately selected etch, so as produce reduced thickness peripheral hinged areas, with the exposed part of the silicon dioxide layer being removed in a subsequent etching step using a different, selective etch. The inventors claimed that their invention produced a single layer, single-crystal, silicon hinge of uniform, continuous material, which enhances the linearity of the diaphragm movement during use and the sensor's sensitivity and accuracy.